a	Cross-section of pipe divided with equal area
A	Cross-section area of the pipe
$A_{ori}$	Area of orifice plate
ANN	Artificial neural network
C	Capacitance
$C_d$	Discharge coefficient
$C_{d,c}$	Discharge coefficient for compressible flows
$C_{d,i}$	Discharge coefficient for incompressible flows
CAPEX	Capital expenditures
$d_{pipe}$	Diameter of pipe
$d_{vc}$	Diameter of pipe at Vena Contracta
D	Internal diameter of the pipe
DP	Differential pressure
ECT	Electrical capacitance tomography
EIT	Electrical impedance tomography
EMT	Electromagnetic tomography
ERT	Electrical resistance tomography
EVT	Electromagnetic velocity tomography
$f_{i\left(x\right)}$	Volume fraction of phase x
FM	Flow meter
$F{r}_g$	Gas densimetric Froude number
$F{r}_l$	Liquid densimetric Froude number
g	Gravitational force
GVF	Gas volume fraction
LBP	Linear back projection
$m_g$	Mass flow rate of the gas
$m_l$	Mass flow rate of the liquid
M	Mass flow rate
MIT	Magnetic induction tomography
MR	Magnetic resonance
MPFM	Multi-phase Flow Meter
MWT	Microwave tomography
N	Number of elements
NIR	Near infra red
OPEX	Operating expenses
P	Pressure
PET	Positron emission tomography
PEPT	Positron emission particle tracking
PT	Pressure and temperature
PW	Produced water
Q	Volumetric flow rate
$Q_x$	Volumetric flow rate of phase x
$U_{sg}$	Superficial gas velocity
$U_{sl}$	Superficial liquid velocity
$v_g$	Instantaneous velocity of gas
$v_i$	Flow velocity in the i-th element
$v_o$	Instantaneous velocity of oil
$v_w$	Instantaneous velocity of water
V	Potential difference
VFM	Virtual Flow Meter
VMS	Virtual Metering System
WLR	Water liquid ratio
$X_{LM}$	Lockhart-Martinelli wet gas parameter
Y	Expansion coefficient
z	Change in elevation
$\alpha$	Gas void fraction
$\beta$	Water fraction
${\beta }_d$	Ratio between diameter of pipe and diameter of orifice
$\chi$	Oil fraction
$\Gamma$	Electrode surface
$\epsilon$	Permeability distribution
${\epsilon }_{r\left(gas\right)}$	Permittivity of gas
${\epsilon }_{r\left(oil\right)}$	Permittivity of oil
${\epsilon }_{r\left(water\right)}$	Permittivity of water
$\varphi$	Electrical potential distribution
${\rho }_g$	Density of gas
${\rho }_l$	Density of liquid
${\rho }_o$	Density of oil
${\rho }_w$	Density of water
$\sigma$	Conductivity distribution